The present invention relates to in situ measurements of earth formations traversed by a well borehole. More particularly, the invention relates to pulsed neutron irradiation measurement techniques for in situ determination of the thermal neutron capture cross sections of earth formations in the vicinity of a borehole passing therethrough.
Pulsed (d,t) sources used in borehole logging produce neutrons which have energies of 14 Mev. These neutrons, when emitted into the borehole, are then moderated by interaction with the nuclei of the materials in the borehole and the surrounding earth formations as they diffuse therethrough. When the neutron energies have moderated to below about 0.05 electron volts, they come into thermal equilibrium with their environment. After reaching this thermal energy range, the neutrons continue diffusing through the formation and borehole until they are captured by nuclei in the constituent materials. The rate at which a zone of material (formation or borehole) captures these thermal neutrons (or more precisely, the probability of capture) is referred to as the macroscopic capture cross-section of the zone. The macroscopic capture cross-section is, in turn, a result of the combined microscopic capture cross-sections of the various constituent elements and materials constituting the zone. The capture cross-section of chlorine for thermal neutrons is considerably higher than that of most other elements commonly encountered in earth formations of interest. Accordingly, thermal neutron macroscopic capture cross-section measurements can give a good indication of the saline content of the fluids in the zone in question. By combining such information about the saline content of the fluids in the pore spaces of an adjacent earth formation with information about the formation water salinity, porosity measurements, and measurements of formation shaliness, information can be derived which can discriminate oil from salt water filled pore spaces in the vicinity of a well borehole.
Since thermal neutrons are absorbed by other materials as well as chlorine, the macroscopic capture cross-section is also responsive to borehole conditions and to the lithology of the formation materials. Prior art thermal neutron cross section methods have therefore typically been structured to try to minimize the borehole effects, for example by delaying the measurements after each neutron pulse so that these effects could then be ignored. That is, since borehole moderation and die away is usually faster than formation moderation and die away, the several measurements which are made are usually delayed until most of the borehole component has had sufficient time to decay. However, some residual borehole component relative to the formation component will usually be present, especially if the borehole contains materials which decay slowly and the formation contains material with a high cross section.
Another disadvantage of delaying the measurement gates to allow the borehole component to dissipate is the loss of important formation and borehole information during this long waiting period. Count rates are high and statistically important during early portions of the thermal neutron cycle, and such prior art delayed measurement methods fail to utilize this information.
Several important improvements in thermal neutron measurements are disclosed in U.S. Pat. Nos. 4,409,481 (Smith, Jr. et al., issued Oct. 11, 1983) and 4,424,444 (Smith, Jr. et al, issued Jan. 3, 1984), both assigned to the assignee of the present invention, the disclosures of which are incorporated herein by reference. In these inventions, at least four, and preferably six, capture gamma ray count rate measurements are made starting immediately after thermalization of the fast neutrons. From these measurements both the borehole thermal neutron lifetime component and the earth formation thermal neutron lifetime component are individually calculated using iterative least squares fitting of the count rate data. Rather than ignoring the borehole component, therefore, it is specifically identified, calculated, and separated from the formation component, substantially improving the accuracy of the formation thermal neutron lifetime measurement. Reference should also made to the following publications wherein additional aspects of these inventions are discussed: Smith, H. D., Jr., Arnold, D. M., and Peelman, H. E., "Applications of a New Borehole Corrected Pulsed Neutron Capture Logging System (TMD)", Paper DD, SPWLA Twenty Fourth Logging Symposium Transactions, June 1983; and Buchanan, J. C., Clearman, D. K., Heidbrink, L. J., and Smith, H. D., Jr., "Applications of TMD Pulsed Neutron Logs in Unusual Downhole Logging Environments", Paper KKK, SPWLA Twenty Fifth Logging Symposium Transactions, June 1984.
The logging systems disclosed in the above-noted U.S. Pat. Nos. 4,409,481 and 4,424,444 are designed to measure .SIGMA..sub.FM, the thermal neutron capture cross section of the formation. As with prior pulsed neutron systems, a 14 MeV pulsed neutron generator source is used to create a time dependent thermal neutron, and hence capture gamma ray, distribution in the vicinity of two gamma ray detectors within the logging tool. The decay rate of the capture gamma radiation measured by the tool is used to obtain .SIGMA..sub.FM, and also a number of other parameters useful in evaluating log quality, borehole conditions, and reservoir performance.
As indicated above, the tool measures count rates in each detector in six different time gates after each neutron burst. These gates, which are dead-time and background count corrected, span the decay from very near the end of one neutron burst until almost the beginning of the next neutron burst. The first two gates are positioned shortly after the end of the burst and detect both formation and borehole count rates. The last four gates are each progressively wider at longer delay times from the neutron burst, and detect primarily formation events. The last gate (Gate 6) is sufficiently delayed from the burst so that a negligible number of counts (generally &lt;3%) in the gate are generated from captures in the borehole fluid.
The six resulting dead-time and background corrected count rates in each detector represent points on the composite formation plus borehole decay curve. These count rates are then adaptively filtered over a short vertical interval in the borehole (one to several feet, depending upon filtering parameters). The main field computer program then uses these six points along the composite formation/borehole decay curve in an iterative least-squares technique to separate the composite curve into the borehole and formation decay components. The computer calculates the formation capture cross section from the slope of the formation decay component. In addition, the computer calculates the borehole capture cross section .SIGMA..sub.BH from the borehole decay component, and calculates the intercepts for each component (i.e., initial values A.sub.FM and A.sub.BH at the end of the neutron burst). This procedure is completed for decay data from both the short-spaced (SS) and the long-spaced (LS) detectors. The resulting .SIGMA.FM-SS and .SIGMA..sub.FM-LS data are as free as possible from borehole effects since, during the computer calculation of .SIGMA..sub.FM, the borehole count rates are essentially "subtracted" from the total observed count rate. The borehole cross section .SIGMA..sub.BH is useful in identifying changes in borehole fluids and composition.
The field computer program also calculates two data quality parameters, .SIGMA..sub.QUAL and R.sub.BH/FM, which can be used to determine how effectively the two exponential program solution matches the observed decay curve. .SIGMA..sub.QUAL is the short spaced detector ratio of the calculated formation component counts in Gate 6 divided by the total observed counts in Gate 6. Since Gate 6 is farthest from the neutron burst (beginning approximately 460 microseconds after burst termination in the preferred prior art embodiment), almost all the counts in Gate 6 should be formation counts. Hence, .SIGMA..sub.QUAL should be just slightly less than or equal to 1.
In the field, however, there can be large washed out (and perhaps cement filled) intervals outside the casing. In this environment, there are actually three significant components--borehole fluid, cement, and formation--in the decay curve. In general, the borehole decay in this case will still be very rapid. However, the borehole decay signal will contain count rate decay information from the borehole fluid, and also from the near borehole just outside the casing. This composite borehole signal will in general be separable from the formation component and from whatever residual cement component is not incorporated in the borehole signal. The remaining formation and residual cement decay rates will usually also be different from one another. However, since the computer program is looking for only one other exponential curve, and not two curves, the program will try to fit just one exponential curve to the composite formation exponential and residual cement components in Gates 3 through 6. Graphically, these formation and cement components usually combine in a concave upwardly shaped curve (on a log scale), while the computer-estimated single least squares fitted exponential line through the data points will be a straight line. In such cases, the computer usually underestimates the counts in the last gate (Gate 6). This results in a .SIGMA..sub.QUAL which is less than 1. (.SIGMA..sub.QUAL can also be useful in determining if filtrate still exists immediately around the wellbore, and it can be used to locate other borehole anomalies, such as packers, gravel packs, and washouts.)
The other data quality parameter, R.sub.BH/FM, is a ratio which measures the total borehole counts calculated for the short spaced detector (=.tau..sub.BJ *A.sub.BH) relative to the formation counts calculated for the short spaced detector (=.tau..sub.FM *A.sub.FM), where .tau. represents the component lifetime. In other words, R.sub.BH/FM indicates the relative contributions in the short spaced detector of the total counts from the borehole and from the formation. R.sub.BH/FM increases as borehole size (including washouts) increases, and also increases as formation porosity decreases. R.sub.BH/FM can also change when borehole salinity changes or when casing changes.
As will be appreciated from the above, necessarily abbreviated discussion of several of the features of the U.S. Pat. Nos. 4,409,481 and 4,424,444 neutron logging methods and apparatus, these represent substantial advances over the prior art. Nevertheless, further improvements could be provided. For example, it would be useful to know which of the two data quality parameters, .SIGMA..sub.QUAL and R.sub.BH/FM, should preferably be used in various borehole conditions. It would be very beneficial to have another method for deriving quality curves for .SIGMA..sub.FM and .SIGMA..sub.BH. And it would be particularly beneficial to have another (perhaps faster and potentially simpler) method for obtaining a parameter indicative of the borehole capture cross section .SIGMA..sub.BH. In fact, with regard to measuring and indicating .SIGMA..sub.BH, it should be particularly noted that changes in the borehole cross section can provide very important data about a well. Therefore, a rapid and simple .SIGMA..sub.BH indicating measurement, even if only qualitative, would be a valuable improvement.